How to output transparent PNG with Pycairo?

Question

Here's my code:

import cairo
import os
from PIL import Image

imagesize = (512,128)
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, *imagesize)

cr = cairo.Context(surface)

cr.select_font_face("Verdana", cairo.FONT_SLANT_NORMAL, cairo.FONT_WEIGHT_NORMAL)
cr.set_font_size(24)
cr.set_source_rgb(1, 1, 1)

...

surface.write_to_png("MyImage.png")

As you can see I'm drawing some white text to this PNG, but the background defaults to an opaque black. How do I make the png transparent so that only the white text is showing?

Answer 1

I was able to setup a transparent background using set_source_rgba() and using 0.0 for the alpha value:

cr.set_source_rgba(0.0, 0.0, 0.0, 0.0) # transparent black
cr.rectangle(0, 0, 512, 128)
cr.fill()

I then also had to make sure to write the text with something like this:

# set writing color to white
cr.set_source_rgb(1, 1, 1)

# write text
cr.move_to(100,50)
cr.show_text("hello")

# commit to surface
cr.stroke()

Here's the full code that works for me:

import os
from PIL import Image

imagesize = (512,128)
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, *imagesize)

cr = cairo.Context(surface)

# paint background
cr.set_source_rgba(0.0, 0.0, 0.0, 0.0) # transparent black
cr.rectangle(0, 0, 512, 128)
cr.fill()

# setup font
cr.select_font_face("Verdana", cairo.FONT_SLANT_NORMAL, cairo.FONT_WEIGHT_NORMAL)
cr.set_font_size(24)
cr.set_source_rgb(1, 1, 1)

# write with font
cr.move_to(100,50)
cr.show_text("hello")

# commit to surface
cr.stroke()

# save file
surface.write_to_png("MyImage.png")